The present invention relates to an image processing apparatus allowing a quality of image of a color cathode-ray tube (color CRT) to be improved when it is used as a picture monitor. More particularly, it relates to the one wherein a high horizontal spatial frequency picture pattern having a peak level and a picture pattern having an edge are extracted respectively, and separate corrections are performed on these picture patterns, thereby improving sharpness of the picture without losing the color saturation.
It is known that, with a picture display apparatus such as the color CRT used as a picture monitor, the waveform becomes dull by passage through signal transmission system of an input picture signal from a signal input unit to a cathode electrode of the color CRT. Further, it is not possible to ensure a sufficient bandwidth for a high input picture signal because of attenuation of a horizontal spatial frequency bandwidth due to the aperture effect in a color CRT display system.
It is known that the sharpness of the image is not sufficient for these reasons. Therefore, for example, when this picture monitor is used as a computer display or the like, it cannot show a small character clearly, so that small character information tends to become difficult to see. Further, particularly for thin line display, a white vertical line on a black background tends to be darker, and a black vertical line on a white background tends to thicken in the horizontal direction.
For this reason, an attempt has been made to sharpen the picture by using the following means in the art. First, for the dullness of the waveform generated in the signal transmission system, correction is made by using a peaking correction circuit. The peaking correction is a processing for compensating the lacking frequency bandwidth by performing a processing for increasing the gain with respect to a given specific horizontal frequency.
For changing the gain by the horizontal frequency, it is recommendable that the impedance determining the gain is allowed to have a frequency characteristic. A specific example of the peaking correction circuit will be described by reference to FIG. 1. A peaking correction circuit 10 is provided between a picture output stage and a cathode electrode of the color CRT, and a grounded emitter amplifier is used as the peaking correction circuit 10, as shown in FIG. 1.
An input picture signal such as a monochrome picture signal SR of R is supplied to a base terminal 12 of an NPN transistor Q. A collector thereof is connected to a power source +Vcc via a resistor 14 and an impedance element 16 which is a serial peaking correction element. Further, an emitter peaking circuit 20 of a resistor 20a and a capacitor 20b may be also connected in parallel to an emitter resistor 18 thereof.
Herein, the high frequency gain of the output picture signal is determined by the impedance element 16, the resistor 20a, and the capacitor 20b. Therefore, utilizing the peaking correction circuit 10 allows the gain for the high frequency component of the input signal frequency to increase, thereby compensating for the loss due to the signal transmission system.
The state of correction by peaking is shown in FIGS. 2A to 2C and FIGS. 3A to 3C. FIGS. 2A to 2C show the case of a white image on a black background, while FIGS. 3A to 3C show the case of a black image on a white background. FIGS. 2A and 3A show ideal waveforms, and FIGS. 2B and 3B respectively show the signal waveforms each deteriorated by passage through the signal transmission system. Then, FIGS. 2C and 3C respectively show the signal waveforms each improved by the peaking processing.
Due to the waveform deterioration in the signal transmission system, for FIG. 2B, white information on a black background darkens, and for FIG. 3B, the line width of black information on a white background increases, as well as the level of the black display portion of the signal increases, resulting in a deterioration in contrast of the detail (the vertical line of a character, or the like) to be expressed. The reduction in contrast is a serious problem particularly for a computer display. However, it is indicated that the reduction in level and the reduction in contrast are both improved by peaking correction as apparent from the waveform processings of FIGS. 2C and 3C.
On the other hand, for the aperture effect of a CRT display system, correction is performed by enhancing the edge of the input picture signal. The edge portion of a picture is enhanced by aperture correction whereby preshoot and overshoot are added to the edge portion, so that the apparent performances of the CRT display system are improved by this enhancement processing.
FIG. 4 shows a specific example of an aperture correction circuit 30. It has a pair of delay circuits 32 and 34 as well as the delay circuit 32 of the first stage receives an input picture signal from an input terminal 36. Its delay output is supplied to an adder 50. Then, an adder 46 adds the ones obtained by multiplying the inputs and outputs of the respective delay circuits 32 and 34 by coefficients ((xe2x88x921) fold and two fold) as shown by means of coefficient multipliers 40, 42, and 44. The one obtained by multiplying the addition output SRe at a coefficient multiplier 48 is supplied to the adder 50, which adds it to the output picture signal.
FIGS. 5A to 5E are waveform diagrams each showing the operations wherein picture signals SRa and SRc respectively preceding and succeeding an input picture signal serving as a reference such as a monochromatic picture signal SRb by one pixel (FIGS. 5A to 5C) are obtained. These are subjected to coefficient multiplication and then passed to the adder 46, so that an edge signal SRe as shown in FIG. 5D is obtained. The coefficient multiplier 48 appropriately adjusts the gain thereof and the one thus adjusted is added to the reference picture signal SRb, thereby obtaining a picture signal SRo whose leading and trailing edges are respectively enhanced as shown in FIG. 5E.
Incidentally, if the peaking correction is performed, it is possible to improve the above described state in which white information on a black background darkens, and it is possible to improve the above described state in which the line width of black information on a white background appears to be large. Further, there are a feature that the deterioration in contrast is also eliminated, and other features.
However, if the peaking correction is performed, ringing occurs. Accordingly, particularly for the case as shown in FIG. 3C, the black information looks whitely edged, so that the quality of the image is largely impaired.
Further, even if ringing roughly has the amplitude characteristic due to the peaking processing, the group delay characteristic is difficult to flatten, and ringing increases with an increase in peaking amount.
Namely, for the peaking correction, the improvement in edge dullness and the inhibition of ringing are not completely compatible. This is because if the peaking amount is decreased, the improvement of the dullness of the edge is insufficient, but it is possible to inhibit ringing: in contrast, if the peaking amount is increased, it is possible to improve the dullness of the edge, but ringing becomes noticeable.
Peaking correction is performed using the resistor, the capacitor, the impedance element, and the like as described above. However, variations in constants of these elements, and variations in value due to the temperature characteristics occur, and hence stable peaking correction is impossible.
On the other hand, in aperture correction, the following problems are presented.
The width of the edge added by aperture correction equals to the unit delay time of the delay circuits 32 and 34 as apparent from FIGS. 5A to 5E. Essentially, the edge is added to a picture, and hence it is constant with respect to the spatial frequency. Namely, it should have a constant width on a screen.
However, in the case where the aperture correction processing is applied to a multi-scan monitor capable of varying the horizontal deflection frequency, when the horizontal deflection frequency is slow, the edge width on a screen narrows, while when the horizontal deflection frequency is rapid, the edge width widens. Too large edge width results in an image which appears to be edged, while too small width results in an image insufficiently corrected.
From these facts, if the aperture correction circuit 30 using the delay circuits 32 and 34 as shown in FIG. 4 is applied to a multi-scan monitor or a CRT monitor handling various display resolutions, it is not possible to obtain a satisfactory image quality.
For solving this problem, it is recommendable that the circuit configuration of FIG. 4 is configured by digital circuits. Further, when the delay circuits 32 and 34 are respectively made up of m flip-flop circuits and the clock thereof is set to be, for example, a pixel clock of the display image, it is possible to change the delay time into m types one pixel by one pixel, thereby solving it.
However, even when the aperture correction circuit 30 is configured as such a digital aperture correction circuit, the following problem remains.
The state of aperture correction by a digital method is described by reference to FIGS. 6A to 6D and FIGS. 7A to 7D. The delay time for aperture correction is defined as being for one pixel (1 dot).
FIGS. 6A to 6D show the case of a white image on a black background, and FIGS. 7A to 7D show the case of a black image on a white background. FIGS. 6A and 7A show ideal luminance waveforms. FIGS. 6B and 7B show the luminance waveforms deteriorated due to the aperture effect, and having lost the sharpness. FIGS. 6C and 7C are respectively luminance waveforms after aperture correction. FIGS. 6D and 7D show the luminance distribution waveforms when the picture signals subjected to aperture corrections have been added to a monitor.
Herein, as shown in FIG. 4, a picture signal is doubled at the coefficient multiplier 44, and the picture signal is multiplied (xe2x88x921) fold at the coefficient multipliers 40 and 42. The multiplications by the coefficients are carried out at all of the edge portions of the input picture signal. However, essentially, the aperture correction processings are not required to be performed on all the picture components with a high horizontal frequency. In other words, in FIGS. 6A to 6D or FIGS. 7A to 7D, when a picture pattern Pa showing a thin line as configured by several pixels (n pixels) and a picture pattern Pb having a given width are present, if aperture correction is performed on the picture pattern Pb, the correction is such that respective edges become sharp. Accordingly, the sharpness is largely improved. Then, the coefficients of the coefficient multipliers 40 to 44 described above are selected such that the edge component can be extracted and improved in sharpness with respect to the picture pattern Pb.
For this reason, if the aperture correction is performed on the picture pattern Pa configured by a pattern for n pixels, and having a level of not less than the peak level, the result is slightly excessive correction, or potentially insufficient correction. This is because it is not possible to discriminate between the narrow-width picture pattern Pa as a thin line having a peak level and the broad-width picture pattern Pb, and to respectively correct them with a conventional aperture correction circuit.
Further, with such the conventional aperture correction circuit, the mixing ratio among R, G, and B is changed. This is because such an operation as to make the mixing ratio among R, G, and B constant is not performed with the conventional circuit. This improper correction causes a large problem that the color saturation of the image is changed.
This will be described by reference to FIGS. 8A to 8E and FIGS. 9A to 9G. For convenience of description, there will be shown the case where a picture signal made up of characters and lines in cyanish color, i.e., in a mixing ratio of R:G:B=0.5:1.0:1.0 on a green background has been inputted.
FIGS. 8A to 8E show a specific example of the aperture correction circuit whereby an edge correction signal is generated from a luminance signal Y, and this is added to each of the monochrome picture signals (primary color signals) R, G, and B to correct the sharpness thereof.
In FIG. 8A, the inputs of R, G, and B are set to be Ri, Gi, and Bi, respectively. For performing the aperture correction, first, the luminance signal Y is calculated from the following equation:
Y=0.30*Ri+0.59*Gi+0.11*Bi
An edge signal Yedge of the luminance signal Y is as shown in FIG. 8B. The edge signal Yedge is multiplied by an aperture correction coefficient K at the coefficient multiplier 48. Assuming that K=0.5, the resulting signal is added to the monochrome picture signals Ri, Gi, and Bi. As a result, corrected monochrome picture signals Ro, Go, and Bo as shown in FIGS. 8C, 8D, and 8E, respectively are obtained.
Herein, considering the timing for performing the edge correction (time point t0), the ratio of inputted monochrome picture signals is:
Ri:Gi:Bi=0.5:1.0:1.0=1:2:2
while the ratio of monochrome picture signals after aperture correction is:
Ro:Go:Bo=0.76:1.26:1.26=1:1.66:1.66
This indicates that the mixing ratio of R, G, and B is changed by performing the aperture correction processing, and the color saturation is changed.
FIGS. 9A to 9G show a specific example of the case where edge signals are generated from the monochrome picture signals R, G, and B themselves, and these are added to respective monochrome picture signals R, G, and B, thereby performing the aperture correction. Therefore, in this case, the aperture correction circuit is required for three channels of R, G, and B.
In this case, edge signals Redge, Gedge, and Bedge (FIGS. 9B, 9C, and 9D) are generated from the monochrome picture signals Ri, Gi, and Bi (FIG. 9A), respectively. The edge signals Redge, Gedge, and Bedge are multiplied by the coefficient K (=0.5) at the coefficient multiplier 48 as shown in FIG. 4, and the multiplication outputs are added to the original monochrome picture signals Ri, Gi, and Bi, respectively, at the adder 50. The addition results are shown in FIGS. 9E, 9F, and 9G.
For example, considering the monochrome picture signal R, the result is:
Ro=Ri+0.5*Redge
Also for other monochrome picture signals G and B, calculation can be performed in the same manner.
Therefore, considering the same timing (timing point t0) as in FIGS. 8A to 8E, the ratio of R, G, and B at this time is:
Ro:Go:Bo=1.0:1.0:2.0
This indicates likewise that the mixing ratio of R, G, and B is changed, and the color saturation is changed between input and output.
Therefore, when this aperture correction processing is applied to a computer display, it becomes impossible to reproduce the hue with fidelity. This indicates that the processing is not suitable for the application requiring high resolution and high fidelity.
This invention proposes an image processing apparatus capable of improving the sharpness without deteriorating the color reproducibility.
The image processing apparatus of this invention comprises pixel judgment means and pixel correction means each receiving digital input picture signal of R, G, and B, respectively, wherein the pixel judgment means includes target pixel detection means for detecting a target pixel having a peak level out of the input picture signal, and edge detection means for detecting an edge from a total of 2n+1 pixels of the target pixel and n pixels preceding and succeeding the target pixel, wherein the pixel correction means includes a correction coefficient selection means for selecting a peak level correction coefficient according to an output from the target pixel detection means, and for selecting an edge correction coefficient according to an output from the edge detection means, and wherein a level of the target pixel is corrected and an edge of the input picture signal is corrected with a pixel of the input picture signal being corrected according to the peak level correction coefficient and the edge correction coefficient, respectively.
In this invention, this image processing apparatus is configured so as to perform the function of aperture correction. It has a means for detecting a picture pattern identified by the signal levels of a total of (2n+1) pixels of the target pixel and at least n pixels (n is not less than 1. In the embodiments, it is assumed that n=1) preceding and succeeding the target pixel, or the signal level difference among these pixels for each of the RGB digital picture signals (monochrome picture signals).
Then, when a picture pattern to be corrected is detected for any one of R, G, and B, the correction determined based on the result obtained by performing such a logical or numerical processing that the detection result is reflected in a single or a plurality of output results is added to respective picture signals of R, G, and B to correct the picture pattern.
A narrow-width picture pattern such as a thin line pattern and a broad-width picture pattern are discriminated between each other, and respectively corrected in this manner. In consequence, it is possible to eliminate the excess or deficiency of the correction amount particularly with respect to the narrow-width picture pattern. Further, performing such an arithmetic processing of the correction amount that the ratio of R, G, and B becomes constant allows the edge correction to perform without changing the mixing ratio of R, G, and B. As a result, it is possible to improve the sharpness of the picture pattern. Since the image processing apparatus in accordance with this invention is based on digital processing, it is capable of performing a stable signal processing without being affected by variations in circuit elements.
As described above, according to this invention, it is so configured that the sharpness is improved by performing the respective individual correction processings on specific picture patterns.
This can improve the deterioration in sharpness when an image having a high horizontal spatial frequency is displayed without causing changes in edging and color saturation, thereby allowing small character information and the like to sharply show.
Further, in accordance with this invention, by appropriately selecting the correction coefficient and the like, it is possible to perform the optimum correction according to the signal characteristics such as frequency and resolution of the input signal, or the performances of each CRT monitor determined by the aperture characteristics of CRT, i.e., the relationship between the beam spot size and the display signal frequency, the frequency characteristics of a picture amplification circuit and the like.